With large-sized refrigeration equipment, if the pressure inside the refrigeration equipment abnormally increases or the temperature of refrigerant increases, the refrigeration equipment itself may be damaged, and high pressure gas may spew out and damage the surroundings. As a mechanism for preventing damage and breakage of refrigerators, large-sized refrigeration equipment is required to have a safety device for controlling the pressure of refrigerant gas in the refrigeration equipment (Rule 7-1-8 of Refrigeration Safety Regulations of the Ministry of Economy, Trade, and Industry of Japan). Such equipment is typically equipped with a safety device such as a fusible plug. Safety devices for refrigeration equipment and their operation are individually designed by each manufacturer in accordance with the refrigerant being used. In the past, CFC (chlorofluorocarbon) refrigerants were the most widely used refrigerants in refrigeration equipment. However, CFC refrigerants have the problem that they can undergo photolysis by the action of ultraviolet light in sunlight in the stratosphere to form active chlorine, which destroys the ozone layer. Therefore, strict restrictions on the use of these refrigerants have been developed on a global level. Accordingly, they are now being replaced by HCFC (hydrochlorofluorocarbon) refrigerants, which are substitutes for CFC refrigerants. HFC (hydrofluorocarbon) refrigerants which have a still smaller ozone-depleting potential have also been developed. Thus, different types of refrigerants are being used in refrigeration equipment.
Fusible plugs used in refrigeration equipment need to be designed based on the refrigerant which is used. When the pressure of refrigerant used in refrigeration equipment increases, the temperature of the refrigerant increases in accordance with Boyle-Charles' law. Therefore, the operating temperature of a fusible plug being used is determined in accordance with the condensation pressure of the refrigerant being used. For example, in the case of refrigerating equipment for air conditioning which employs R22 (HCFC 22) (which is the HCFC refrigerant which currently is most in demand) as a refrigerant for refrigeration equipment, the condensation pressure is 1.94 MPa, and the critical temperature of R22 is 96.2° C. Therefore, the operating temperature of a fusible plug is designed to be approximately 95-100° C.
The critical temperature varies with the refrigerant being used. When the refrigerant is changed, it is necessary to redesign the operating temperature of a fusible plug. When using R404A, which is a HFC refrigerant used as a substitute refrigerant having a small ozone-depleting potential, it has a condensation pressure of 3061 KPa, and its critical temperature is 71.6° C. Therefore, a fusible plug having a design temperature of approximately 66-70° C. should be used.
Fusible plugs for refrigeration equipment use low melting point solder alloys, and the solder alloys which have been used are those containing Pb or Cd which is harmful. When R22 is used as a refrigerant, since the design temperature of a fusible plug is 96° C., a Sn-52Bi-32Pb alloy (eutectic at 96° C.) has been used. When R410a is used as a refrigerant, the design temperature of a fusible plug becomes 71-75° C., and a solder alloy such as Sn-50Bi-10Cd-26.7Pb (solidus temperature of 69° C., peak temperature of 76° C., liquidus temperature of 81° C.) has been used.
In general, fusible plugs are recovered together with refrigeration equipment. When refrigeration equipment is discarded, it is of course necessary to treat the equipment in accordance with laws and regulations. Particularly in recent years, active efforts are being made to protect the global environment, and there is a tendency to exclude harmful elements from parts used in equipment such as refrigeration equipment. In particular, as Cd and Pb have a harmful effect on the human body, they have become the subject of regulations.
Known alloys for fusible plugs which do not contain harmful elements such as Cd and Pb include alloys of two or more elements selected from Sn, Bi, In, Zn, and Ga (JP 2002-115940 A1), low melting point alloys for fusible plugs of a tin-indium-bismuth alloy wherein when the composition is Sn: X wt %, In: Y wt %, and Bi: Z wt %, then X+Y+Z=100 and 4≤X≤10 and 56≤Y≤63 (JP2001-214985 A1), and fusible alloys of bismuth, indium, and tin containing fine metal particles added thereto (JP 2003-130240 A1).
Patent Document 1: JP 2002-115940 A1
Patent Document 2: JP 2001-214985 A1
Patent Document 3: JP 2003-130240 A1